Work in a school district long enough (say, twenty minutes or so) and you'll be told that something is "legally required" when it is actually District policy.
Ask a few questions and you may find that out. Generally though, no one will be able to point you to the policy. It is one of those fantastic school urban legends, the unwritten policy.
Spend another twenty minutes working in a school district and you will find that as unwritten and unjustifiable as they may be, these unwritten policies are handed down from District bigwig to content specialist to administrator to teacher in hushed tones and serious voices. This is received knowledge with mystical, near-religious status.
Sadly, it's often entirely made up, unethical, or of dubious legality. These are the real reasons the policies are unwritten. That said, they may as well be written on an ancient stele or tattooed on District employees, because they are very effective at keeping cash from being spent.
Along these lines: it is not actually the case that federal - no wait state, er, I mean...well, some law - bars direct services from being offered under a 504 plan. Nor is it in fact the case that children with a 504 cannot be assessed for said services unless they also have an IEP.
The actual issue here is that the services provided under a 504 plan are paid for by a district, not by state or federal dollars. So ultimately, this policy exists to spare the District from providing expensive services to children for which they will not be reimbursed.
Whether this enables the district to provide that Free and Appropriate Public Education, however...well, let's just note that lawyers are standing by to collect retainers on this issue.